The third-order nonlinear optical coefficient X.sup.(3) is the key parameter for evaluating materials to be used in nonlinear optical devices. Both real and imaginary parts of the refractive index change significantly with the intensity of light only when the value of X.sup.(3) is sufficiently large. Optical devices which are to be controlled by an external light or electrical field must have a large value of X.sup.(3) for practical application.
Some classes of semiconductor devices are known to have large X.sup.(3) and can be integrated with conventional semiconductor optical devices.
The literature concerning these techniques includes the following:
(1) S. Y. Yuen and P. A. Wolff: Technical Digest, pp. 150-153 of Nonlinear Optical Properties of Materials Topical Meeting; August, 1988, Troy, N.Y. PA1 (2) D. A. B. Miller, C. T. Seaton, M. E. Price, and S. D. Smith: Phys. Rev. Lett. 47, 197 (1981). PA1 (3) C. K. N. Patel, R. E. Slusher, and P. A. Fleury: Phys. Rev. Lett. 17, 1011-1017 (1966). PA1 (4) D. A. B. Miller, D. S. Chemla, D. J. Eilenberger, P. W. Smith, A. G. Gossard, and W. Wiegmann: Appl. Phys. Lett., 42, 925 (1983). PA1 (5) H. M. Gibbs, S. S. Tarng, J. L. Jewell, D. A. Weinberger, and K. Tai: Appl. Phys. Lett., 41, 221-222 (1982). PA1 (6) A. Honold, L. Schultheis, J. Kuhl, and C. W. Tu: Technical Digest of 16th International Conf. on Quantum Electronics, Tokyo (1988). PA1 (7) T. Ishihara, J. Takahashi, and T. Goto: Solid State Communication, 69, 933 (1989). PA1 (8) E. Hanamura: Nato workshop on "Optical Switching in Low-dimensional Systems," October (1988), Marballa, Spain. PA1 (9) Yu. I. Dolzhenko, T. Inaba and Y. Maruyama: Bull. Chem. Soc. Japan, 59, 563 (1986).
In a narrow-band-gap semiconductor, band filling (Ref. 2) and nonparabolicity of the conduction band (Ref. 3) cause extremely large nonlinearity (e.g. X.sup.(3) =3.times.10.sup.-1 for a single InSb crystal). However, in practice, because the nonlinearity is significant at longer wavelengths (5 .mu.m to 10 .mu.m), this class of semiconductor is not generally considered for use in optical data processing devices.
In order to achieve higher packing density, it is desirable to find other mechanisms for achieving a large value of X.sup.(3) at shorter wavelengths. The excitonic transition of semiconductor quantum wells is very promising (Refs. 4, 5). The large value of X.sup.(3) is a the result of saturation absorption at exciton resonances. Since the contribution of excitonic transitions (compared to that of inter-band transitions) is larger when the band gap is wide, this mechanism is more favorable at shorter wavelengths. The excitonic process giving rise to large X.sup.(3) is common to bulk crystal systems, but the excitonic transition of a modulated structure or quantum well system (when referred to hereafter, the latter includes the former) is more significant because of the enhancement by the quantum confinement effect.
Although this system is far better than bulk crystal systems, it exhibits several practical problems which must be solved.
The exciton binding energy in a quantum well is still low (approximately 10 meV). Therefore, phonons at room temperature decrease the number of excitons, and it is necessary to operate the device below the temperature of liquid nitrogen to obtain strong absorption.
The system response time is long because of the long lifetime of the free carriers. It is therefore difficult to construct a fast device.
Manufacturing a semiconductor quantum well requires expensive equipment and complicated manufacturing processes such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD).
It is still not possible to make an ideal quantum well with completely flat structure on an atomic scale. Fluctuations in the epitaxial growth process cause irregularity in the size of the quantum well. An exciton lifetime of 2.8 ps is expected for the ideal quantum well system. In practice a lifetime of 180 ps has been observed for an actual system (Ref. 6).
In order to obtain a large value of X.sup.(3) and a fast response time, a multi-quantum well semiconductor system without any irregularity in the well structure is required. In this regard, a two-dimensional perovskite semiconductor crystal, (C.sub.10 H.sub.21 NH.sub.3).sub.2 PbI.sub.4 in a state exhibiting an intercalation of the semiconductor layer and the organic material layer, is drawing attention.
The crystal structure of (C.sub.10 H.sub.21 NH.sub.3).sub.2 PbI.sub.4 is shown in FIG. 6. As shown in this figure, each mono-layer consisting of Pb.sup.2+ and I is sandwiched by two layers of alkyl-NH.sub.3. The N atom, which is adjacent to the Pb.sup.2+ ion, forms a ligand field, thereby determining the wavelength of the optical transition of Pb.sup.2+. What has been described above is an ideal quantum well structure in the form of intercalation of a semiconductor and an organic material and having a constant well thickness of 6.24 .ANG. throughout the crystals. Moreover, the binding energy of the exciton in this quantum well is quite large: the observed value is 370 meV (which is twelve times as large as that of the bulk crystal of PbI.sub.2), and thus a strong absorption peak can be observed even at room temperature (300.degree. K.), as shown by curve (a) of FIG. 9.
The two-dimensional perovskite semiconductor crystal looks extremely promising as a nonlinear optical device. However, a problem lies in the difficulty of making a large enough crystal for device application, if the previously reported crystal growth method is used (Ref. 9). It takes one to two months to obtain a single crystal having dimensions of 2.times.2.times.0.1 mm.sup.3 when using the silica-gel technique. Another problem is the difficulty in controlling the crystal size (especially the crystal thickness) when this technique is used.